Oxide evaporation material and high-refractive-index transparent film

ABSTRACT

An oxide evaporation material in the present invention comprises a sintered body containing indium oxide as a main component thereof and cerium with the Ce/In atomic ratio of more than 0.110 and equal to or less than 0.538, and has an L* value of 62 to 95 in the CIE 1976 color space. The oxide evaporation material with the L* value of 62 to 95 has an optimal oxygen amount. Accordingly, even when a small amount of oxygen gas is introduced into a film-formation vacuum chamber, a high-refractive-index transparent film having a refractive index of 2.15 to 2.51 at a wavelength of 550 nm, a low resistance, and a high transmittance in the visible to near-infrared region is formed by vacuum deposition methods. Since the introduced oxygen gas amount is small, the difference in composition between the film and the evaporation material is made small.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improvements in: an oxide evaporation material used when a transparent conducting film and a high-refractive-index transparent film are formed by any of various vacuum deposition methods such as electron beam deposition, ion plating, and high-density plasma-assist evaporation; and a high-refractive-index transparent film formed using the oxide evaporation material.

2. Description of the Related Art

A transparent conducting film has a high conductivity and a high transmittance in the visible region. By taking advantage of these characteristics, the transparent conducting film is utilized as an electrode or the like of solar cells, liquid crystal display elements, and various other light receiving elements. Furthermore, by taking advantage of the reflection and absorption characteristics in the near-infrared region, the transparent conducting film is utilized also as: a heat-ray reflection film used for window glasses of automobiles, architectures, and the like; a variety of antistatic films; and an anti-fogging transparent heater for refrigerated showcases or the like.

Generally, the widely used transparent conducting films are formed of: tin oxide (SnO₂) containing antimony or fluorine as a dopant; zinc oxide (ZnO) containing aluminum, gallium, indium, or tin as a dopant; indium oxide (I₂O₃) containing tin, tungsten, or titanium as a dopant; and the like. Particularly, an indium oxide film containing tin as a dopant, i.e., an In₂O₃—Sn film is referred to as an indium tin oxide (ITO) film, and is industrially widely used to date because a low-resistance transparent conducting film is easily obtained.

As to a method for forming such transparent conducting films, generally used are vacuum deposition methods, sputtering methods, and methods involving application of a coating for forming a transparent conducting layer. Above all, the vacuum deposition methods and the sputtering methods are effective methods for a case where a material having a low vapor pressure is used or where precise film thickness control is required. Moreover, these methods are very simple in operation and thus industrially useful. As the vacuum deposition methods are compared with the sputtering methods, the vacuum deposition methods are capable of forming a film at a faster rate and thus superior in productivity.

In the vacuum deposition methods, generally, a solid or liquid evaporation source is heated in a vacuum of approximately 10⁻³ to 10⁻² Pa and temporarily decomposed to gas molecules or atoms which are then condensed on the surface of a substrate as a thin film again. General methods for heating an evaporation source are a resistance heating method (RH method) or an electron-beam heating method (EB method, electron beam deposition). Additionally, methods utilizing laser light, high-frequency induction heating, and the like are also available. Furthermore, flash evaporation, arc plasma deposition, reactive evaporation method, and the like are also known. The vacuum deposition method includes these methods.

The electron beam deposition has been historically frequently utilized for depositing an oxide film such as ITO. Specifically, an ITO oxide evaporation material (may also be called an ITO tablet or an ITO pellet) is used as the evaporation source, and an O₂ gas serving as the reactive gas is introduced into a film-formation chamber (chamber). Thermal electrons jumped off from a thermal-electron generating filament (mainly a W wire) are accelerated by an electric field and radiated to the ITO oxide evaporation material. The oxide evaporation material is locally heated at the radiated area thereof, and evaporated and deposited to a substrate. Meanwhile, activated reactive evaporation (ARE method) is also a useful method for ITO film formation. In this method, a plasma is generated using a thermal electron emitter or RF discharge, and an evaporation material and a reactive gas (O₂ gas, or the like) are activated by this plasma, thereby forming a low-resistance film on a low-temperature substrate. Furthermore, high-density plasma-assist evaporation (HDPE method) using a plasma gun also has been revealed to be an effective method for ITO film formation, and begun to be industrially widely used recently [see “Vacuum,” Vol. 44, No. 4, 2001, pp. 435-439 (hereinafter, “Non-Patent Document 1”)]. This method utilizes an arc discharge using a plasma generator (plasma gun). The arc discharge is maintained between a cathode inside the plasma gun and a crucible (anode) of an evaporation source. Electrons emitted from the cathode are guided by a magnetic field, concentrated and radiated to a local area of an ITO oxide evaporation material put in the crucible. An evaporant is generated from the area that is locally heated by the radiation of the electron beams, and deposited to a substrate. The vaporized evaporant and an introduced O₂ gas are activated in this plasma, so that an ITO film having favorable electrical characteristics can be formed. Meanwhile, as another classification of these various vacuum deposition methods, those involving ionization of an evaporation material and a reactive gas are collectively referred to as ion plating (IP method). Ion plating is effective as a method to obtain an ITO film having a low resistance and a high transmittance [see “Technology of transparent conductive film,” Ohrmsha, Ltd., 1999, pp. 205-211 (hereinafter, “Non-Patent Document 2”)].

In any type of solar cell using the transparent conducting film, the transparent conducting film is essential for an electrode on the front side from which light enters the cell. As the transparent conducting film, the aforementioned ITO film, a zinc oxide film doped with aluminum (AZO), or a zinc oxide film doped with gallium (GZO) has been conventionally utilized. These transparent conducting films are required to have such characteristics as a low resistance and a high transmittance of visible light. As methods for forming these transparent conducting films, the above-described vacuum deposition methods such as ion plating and high-density plasma-assist evaporation are used.

Meanwhile, the above-described thin film containing indium oxide, tin oxide, or zinc oxide as the main component is utilized also as an optical film. Such a thin film is a high-refractive-index material having a refractive index of 1.9 to 2.1 in the visible region, and is capable of demonstrating an optical interference effect when formed into a laminate in combination with a low-refractive-index film such as a silicon oxide film and metal fluoride film which have a refractive index of 1.3 to 1.5 in the visible region. Specifically, when the laminate is formed under the precise control of the thickness of each film, the laminate can have a reflection preventing effect or a reflection enhancing effect in a particular wavelength region. For this usage, a high-refractive-index film having a higher refractive index is more useful since a stronger interference effect can be easily obtained.

Japanese Patent Laid-open Application No. 2005-242264 (hereinafter, “Patent Document 1”) states that an indium oxide film containing cerium has a higher refractive index than the aforementioned tin oxide film, zinc oxide film, and the like. Patent Document 1 proposes an example of using the film as an optical film. Furthermore, Japanese Patent No. 3445891 and Japanese Patent Laid-open Application No. 2005-290458 (hereinafter, respectively “Patent Documents 2 and 3”) each propose techniques related to: a sputtering target material made of indium oxide containing cerium (In—Ce—O); and a transparent conducting film obtained by sputtering the sputtering target material. Moreover, Patent Document 2 states that since the indium oxide-based transparent conducting film containing cerium proposed therein poorly reacts with Ag, a transparent conducting film having a high transmittance and excellent heat resistance can be formed when the indium oxide-based transparent conducting film is stacked on a Ag-based ultra-thin film. Patent Document 3 states that a film having excellent etching characteristics is obtained.

When a thin film such as a transparent conducting film is formed by vacuum deposition methods such as electron beam deposition, ion plating, and high-density plasma-assist evaporation, a sintered body small in size (for example, having a diameter of approximately 10 to 50 mm, a height of approximately 10 to 50 mm, and a cylindrical shape) is used as an oxide evaporation material in the vacuum deposition methods. This limits the amount of film that can be formed from a single oxide evaporation material. Moreover, when the remaining amount of oxide evaporation material is decreased as the consumed amount is increased, the following procedure has to be performed: terminating the film formation; introducing air into the film-formation chamber in the vacuum state for replacement with a fresh oxide evaporation material yet to be used; and evacuating the film-formation chamber again. This consequently lowers the productivity.

Essential techniques adopted in mass production of transparent conducting films by the vacuum deposition methods such as electron beam deposition, ion plating, and high-density plasma-assist evaporation include a method of continuously supplying the oxide evaporation materials. Non-Patent Document 1 describes an example of such a continuous supply method. In the continuous supply method, cylindrical oxide evaporation materials are housed in series inside a cylindrical hearth, and are sequentially pushed out and continuously supplied while the height of the sublimation surface is kept the same. The continuous supply method of an oxide evaporation material enables mass production of transparent conducting films by the vacuum deposition methods.

An indium oxide film containing cerium is normally formed by sputtering methods, as proposed in Patent Documents 2 and 3. However, in recent years, various vacuum deposition methods that are advantageous in terms of productivity are highly demanded for the film formation.

However, there are few technique related to an oxide evaporation material for stably forming an indium oxide film containing cerium by vacuum deposition methods. A technique for manufacturing a sintered body of a sputtering target has been adopted so far to manufacture an oxide evaporation material.

In a case of forming a high-refractive-index transparent film having a low resistance and a high transmittance by any of various vacuum deposition methods such as electron beam deposition, ion plating, and high-density plasma-assist evaporation, using an oxide evaporation material manufactured by the technique adopted so far, a large amount of oxygen gas needs to be introduced into a film-formation vacuum chamber during the film formation. This brings about problems mainly described below.

First, the transparent film and the oxide evaporation material greatly differ in composition from each other, making it difficult to design the composition of the transparent film. This is because, generally, when a larger amount of oxygen is introduced into a film-formation vacuum chamber, the difference in composition between a transparent film and an oxide evaporation material is likely to increase. In the mass production process of films, the amount of oxygen in a film-formation vacuum chamber also tends to vary. Due to the variation in the oxygen amount, the compositions of the films are likely to differ from one another, resulting in the variation of the film characteristics.

Moreover, when the oxygen amount is increased, film formation by reactive evaporation using an oxygen gas causes problems that not only does the film density decrease, but also the adhesive force of the film to the substrate weakens, for example. These problems occur for the following reason. Specifically, when evaporated metal oxide is oxidized before reaching the substrate, the energy is lost. Thus, an increase in the oxidation ratio makes it difficult to obtain a dense film strongly adhering to the substrate.

Furthermore, suppose a case where a high-refractive-index transparent film is formed on a substrate covered with a metal film or an organic film having a surface that can be oxidized easily. In this case, if a large amount of oxygen gas is introduced into a film-formation vacuum chamber, the substrate surface is oxidized before the film formation. This hinders fabrication of a high-performance device. This tendency becomes more noticeable as the temperature of the substrate during the film formation is higher. For example, assume a case where an optical thin film laminate having an interference effect by stacking an ultra-thin metal film such as Ag and a high-refractive-index transparent film. In this case, if the high-refractive-index transparent film is formed on the surface of the ultra-thin metal film with a large amount of oxygen introduced, the ultra-thin metal film is oxidized, hindering formation of a favorable optical thin film laminate.

SUMMARY OF THE INVENTION

The present invention has been made by focusing on such problems. What it aims is to provide an oxide evaporation material mainly containing indium oxide to which cerium is added, the oxide evaporation material enabling stable formation of a high-refractive-index transparent film having a low resistance and a high transmittance even when a small amount of oxygen is introduced during the film formation. Together provided is a high-refractive-index transparent film formed using the oxide evaporation material.

Specifically, an oxide evaporation material according to the present invention comprises a sintered body containing: indium oxide as a main component thereof; and cerium, wherein a cerium content is more than 0.110 and equal to or less than 0.538 in a Ce/In atomic ratio, and an L* value in a CIE 1976 color space is 62 to 95.

A high-refractive-index transparent film according to the present invention is formed by electron beam deposition, ion plating, or high-density plasma-assist evaporation, using an oxide evaporation material comprising a sintered body containing: indium oxide as a main component thereof; and cerium, the oxide evaporation material having a cerium content of more than 0.110 and equal to or less than 0.538 in a Ce/In atomic ratio and an L* value of 62 to 95 in a CIE 1976 color space, the high-refractive-index transparent film having a cerium content of more than 0.110 and equal to or less than 0.538 in the Ce/In atomic ratio, and having a refractive index of 2.15 to 2.51 at a wavelength of 550 nm.

The oxide evaporation material according to the present invention with the L* value in the CIE 1976 color space of 62 to 95 has an optimal oxygen amount. Accordingly, the use of this oxide evaporation material enables formation by vacuum deposition methods of a high-refractive-index transparent film having a low resistance and a high transmittance in the visible to near-infrared region, even when a small amount of an oxygen gas is introduced into a film-formation vacuum chamber. Moreover, since the amount of the oxygen gas introduced into the film-formation vacuum chamber is small, the difference in composition between the film and the oxide evaporation material is made small. This not only facilitates formation of a targeted film composition, but also reduces the variations in the compositions and the characteristics of films formed in large quantities. Furthermore, since the amount of an oxygen gas introduced into the film-formation vacuum chamber is small in the film formation, damage to the substrate by the oxygen gas can be reduced.

In addition, the use of the oxide evaporation material according to the present invention has an effect of stably forming the high-refractive-index transparent film by vacuum deposition methods that are more advantageous to high-speed film formation than sputtering methods.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are described below in detail.

(1) Oxide Evaporation Material

An oxide evaporation material of the present invention has a composition containing indium oxide as a main component thereof and cerium with the Ce/In atomic ratio of more than 0.110 and equal to or less than 0.538. A high-refractive-index transparent film formed by a vacuum deposition method using the oxide evaporation material of the present invention has a composition very similar to the composition of the oxide evaporation material. Thus, the composition of the film to be formed also contains indium oxide as a main component thereof and cerium with the Ce/In atomic ratio of more than 0.110 and equal to or less than 0.538. The reason that cerium is contained in the aforementioned ratio is that cerium in the ratio can increase the refractive index of the indium oxide film. Specifically, if the cerium content (Ce/In atomic ratio) in the composition of the oxide evaporation material is lower than 0.110, the refractive index is lower than 2.15 at a wavelength of 550 nm, and a transparent film having a high refractive index cannot be obtained. Meanwhile, if the cerium content (Ce/In atomic ratio) in the composition of the oxide evaporation material exceeds 0.538, the oxide evaporation material (tablet) contains cerium excessively. Accordingly, a sintered body having a practical strength cannot be obtained, and also the conductivity of the tablet itself is impaired, making it difficult to stably use the tablet in electron beam deposition or the like. Thus, such cerium contents are not preferable.

The most distinguishing characteristic of the oxide evaporation material according to the present invention is that the oxide evaporation material is specified by an L* value in a CIE 1976 color space. Herein, the CIE 1976 color space is a color space endorsed by the international commission on illumination (CIE) in 1976, and represents a color by coordinates in the uniform color space with lightness L* and chromatic indexes a* and b*. The CIE 1976 color space is also abbreviated as the CIELAB. The L* value represents lightness: L*=0 indicates black, while L*=100 indicates diffuse white. Moreover, a negative value of a* indicates a color approximate to green, while a positive value thereof indicates a color approximate to magenta. A negative value of b* indicates a color approximate to blue, while a positive value thereof indicates a color approximate to yellow.

The colors of the surface and the inside of a sintered body of the oxide evaporation material according to the present invention are preferably the same when specified by an L* value in the CIE 1976 color space. In the present invention, if colors are different between the outermost surface and the inside of the sintered body of the oxide evaporation material, the L* value is determined based on the inside of the sintered body.

According to the experiments by the present inventors, when the inside of the oxide evaporation material has an L* value of 62 to 95, a high-refractive-index transparent film having both a high conductivity and a high transmittance in the visible to near-infrared region can be obtained, even when a small amount of oxygen is introduced into a film-formation vacuum chamber. Moreover, the more whitish, the larger the L* value; in contrast, the more blackish, the smaller the L* value. It is thought that: the L* value of an oxide evaporation material is correlated with the amount of oxygen contained in the oxide evaporation material; the larger the L* value, the higher the oxygen content; and the smaller the L* value, the lower the oxygen content. The present inventors have made experiments on forming a high-refractive-index transparent film by vacuum deposition methods using oxide evaporation materials of various L* values under different formation conditions. In the experiments, the larger the L* value, the smaller the optimal amount of oxygen introduced during the film formation (i.e., the amount of oxygen to obtain a film having a low resistance and a high transparency). This is because an oxide evaporation material having a larger L* value supplies larger amount of oxygen therefrom. Meanwhile, it was shown that the difference in composition between the film and the oxide evaporation material tends to be large when a large amount of oxygen is introduced. Thus, the larger the L* value, the smaller the difference in composition.

Note that the oxide evaporation material according to the present invention has a high conductivity, and the conductivity of the oxide evaporation material is dependent on not only the oxygen content, but also the density, the crystal diameter, and the cerium-doping efficiency. Accordingly, the conductivity of the oxide evaporation material does not correlate with the L* value on a 1:1 basis.

When vacuum deposition is performed on the oxide evaporation material according to the present invention containing indium oxide as the main component and cerium, particles are evaporated mainly in the form of In₂O_(3-x) and CeO_(2-x) from the oxide evaporation material. The particles absorb oxygen in a chamber while reacting with oxygen, and reach a substrate for film formation. When the particles are to reach the substrate and are deposited onto the substrate, the energy of the evaporated particles serves as a driving source for the substance movement, contributing to densification of the film and to enhancement of the adhesive force to the substrate. When the smaller the L* value of the oxide evaporation material, the smaller the amount of oxygen in the oxide evaporation material, and the larger the amount of oxygen deficiency in the evaporated particles. For this reason, a large amount of oxygen needs to be introduced into the vacuum chamber so as to increase the proportion of the particles that are oxidized before reaching the substrate. However, since the energy of evaporated particles is consumed when the particles are oxidized during the flying, this makes it difficult to obtain a dense film having a strong adhesive force to the substrate in reactive film formation with the amount of oxygen introduced being large. In contrast, in the reactive-evaporation film formation with the lowest possible amount of oxygen gas introduced, a film having high adhesion and high density characteristics is easily obtained. Such a film formation can be performed with the oxide evaporation material of the present invention.

Here, the L* value of smaller than 62 is not preferable. This is because the amount of oxygen in such an oxide evaporation material is too small. To obtain a film having a low resistance and a high transparency, the optimal amount of oxygen introduced into the film-formation vacuum chamber has to be large. This causes problems such as an increased difference in composition between the film and the oxide evaporation material as well as a great damage to the substrate during the film formation. Meanwhile, if the L* value exceeds 95, the amount of oxygen contained in the oxide evaporation material is too large. Thus, evaporation or sublimation is difficult and high film formation speed cannot be obtained.

Meanwhile, Patent Document 3 proposes a sputtering target that is a sintered body of indium oxide containing cerium. The sintered body of indium oxide containing cerium produced according to the method described therein has a low L* value of 35 to 55. Thus, if such a sintered body is used as an oxide evaporation material, a large amount of oxygen needs to be introduced into a film-formation vacuum chamber to obtain an optimal film, causing the above-described problems. Accordingly, such a sputtering target cannot achieve the object of the present invention.

In this respect, the oxide sintered body to be deposited (oxide evaporation material) of the present invention having an L* value of 62 to 95 cannot be produced according to the conventional technique for producing a sputtering target. An oxide evaporation material having an appropriate oxygen amount (or oxygen deficiency amount) suitable for use in mass production by vacuum deposition methods can be manufactured by a method as follows.

Specifically, powders of indium oxide and cerium oxide are used as the raw materials of the oxide sintered body containing indium oxide as the main component and cerium. These powders are mixed and molded to form a compact, which is fired at high temperatures followed by reaction and sintering to thus produce the oxide sintered body. The powders of indium oxide and cerium oxide are not dedicated, and conventionally used raw materials can be used for the oxide sintered body. The average particle diameter of the powder used is 1.5 μm or smaller, preferably 0.1 to 1.1 μm.

As the method for mixing raw-material powders when the oxide sintered body is produced, a ball-mill mixing method is generally utilized. Such a method is also effective for producing the sintered body of the present invention. A ball mill is a device in which hard balls (having a ball diameter of 10 to 30 mm) such as ceramic and a material powder are put in a container and rotated for grinding and mixing the material to make a fine powder mixture. The ball mill (grinding medium) used has: a shell made of steel, stainless steel, nylon, or the like; and a lining made of alumina, magnetic material, natural silica, rubber, urethane, or the like. Examples of the ball include an alumina ball containing alumina as a main component thereof, natural silica, a nylon ball with iron core, and a zirconia ball. There are wet and dry grinding methods which are widely utilized for mixing and gridding a raw-material powder to obtain a sintered body.

Alternatively, a bead mill and a jet mill are also effective methods other than the ball mill mixing. Particularly, since the cerium oxide powder is a hard material, these methods are considerably effective when a raw material having a larger average particle diameter is used or when grinding and mixing need to be performed in a short period. In the bead mill, a container called a vessel is filled with beads (grinding media, having a bead diameter of 0.005 to 3 mm) by 70 to 90%, and a rotation shaft at the center of the vessel is rotated at a circumferential speed of 7 to 15 m/second to transmit the motion to the beads. Here, a slurry obtained by mixing a liquid with a material to be ground such as a raw-material powder is fed to the vessel with a pump. The beads collide with the slurry for pulverization and dispersion. In the case of the bead mill, the efficiency is increased by reducing the bead diameter in accordance with the material to be ground. Generally, the bead mill can achieve pulverization and mixing at an acceleration approximately 1000 times faster than that of the ball mill. The bead mill having such a mechanism is called by various names. For example, known are sand grinder, aquamizer, attritor, pearl mill, abex mill, ultra visco mill, dyno mill, agitator mill, co-ball mill, spike mill, SC mill, and so forth. Any of these can be used in the present invention. Meanwhile, the jet mill is a method in which high-pressure air or vapor jetted approximately at a speed of sound from a nozzle collides, as a high-speed jet, with a material to be ground such as a raw-material powder to create particle-on-particle impact, thereby grinding the material into fine particles.

As described above, first, an indium oxide powder and a cerium oxide powder are put into a ball mill pot in a desired ratio, and subjected to dry or wet mixing. Thus, a powder mixture is prepared. To obtain the oxide sintered body of the present invention, the blending ratio of the raw-material powders is adjusted in such a manner that the indium and cerium contents are more than 0.110 and equal to or smaller than 0.538 in the Ce/In atomic ratio.

Water and organic materials such as a dispersant and a binder are added to the powder mixture thus prepared to form a slurry. The viscosity of the slurry is preferably 150 to 5000 cP, more preferably 400 to 3000 cP.

Next, the obtained slurry and beads are put in a container of a bead mill for processing. Examples of the bead material include zirconia, alumina, and the like. From the viewpoint of wearing resistance, zirconia is preferable. The diameter of the bead is preferably 1 to 3 mm from the viewpoint of grinding efficiency. The number of passes may be one, preferably two or more times, and a sufficient effect is obtained when the number of passes is five or less. The processing time is preferably 10 hours or shorter, further preferably 4 to 8 hours.

By such processing, the indium oxide powder and the cerium oxide powder in the slurry are ground and mixed well.

Next, the slurry thus processed is molded. As the molding method, any of casting molding and press molding can be adopted. When casting molding is performed, the obtained slurry is poured into a die for casting molding to form a green compact. The time from the bead mill processing to the casting is preferably within 10 hours. Thereby, the resultant slurry can be prevented from exhibiting thixotropy. Meanwhile, when press molding is performed, a binder such as polyvinyl alcohol, and the like are added to the obtained slurry. After water content adjustment when necessary, the mixture is dried with a spray dryer or the like, and granulated. The obtained granulated powder is filled into a die of a predetermined size, and then subjected to uniaxial press molding with a press at a pressure of 100 to 1000 kg/cm² to form a green compact. The thickness of the green compact at this time is preferably set to a thickness enough to obtain a sintered body of a predetermined size, in consideration of shrinkage in the subsequent firing process.

The oxide sintered body of the present invention can be obtained by adopting pressureless sintering using the above-described green compact formed from the powder mixture. An oxide sintered body is obtained by firing in this pressureless sintering as follows.

First, the obtained green compact is heated at 300 to 500° C. for approximately 5 to 20 hours to remove the binder. After that, the temperature is increased for sintering. The rate of temperature rise is 150° C./hour or lower, preferably 100° C./hour or lower, and further preferably 80° C./hour or lower, to effectively release the void inside the compact to the outside. The sintering temperature is 1150 to 1300° C., preferably 1200 to 1250° C. The sintering time is 1 to 20 hours, preferably 2 to 5 hours. It is important that the binder-removing process to the sintering process be performed while oxygen is introduced into a furnace at a rate of 5 liters/minute or more per 0.1 m³ of the furnace capacity. Oxygen tends to dissociate from a sintered body at 1150° C. or higher, so that the sintered body is likely to be in an excessive reduction state. The reason that oxygen is introduced during the sintering process is to avoid this phenomenon. Once a sintered body having oxygen deficiency excessively introduced therein is formed in this process, it is difficult to optimally adjust the oxygen deficiency amount of the sintered body in the subsequent oxygen-amount adjustment process. If the firing temperature exceeds 1300° C., oxygen rapidly dissociates even in an oxygen atmosphere as described above, so that the sintered body is likely to be in an excessive reduction state. Thus, the temperature exceeding 1300° C. is not preferable for the same reason. Meanwhile, if the firing temperature is lower than 1150° C., the temperature is too low to obtain a sintered body having a sufficient strength by the sintering. Thus, the temperature lower than 1150° C. is not preferable.

After the sintering, the oxygen-amount adjustment process is performed on the sintered body. It is important that the oxygen-amount adjustment process be performed at a heating temperature of 900 to 1100° C., preferably 950 to 1050° C., for a heating time of 10 hours or longer. During the cooling to the heating temperature of the oxygen-amount adjustment process, oxygen is continuously introduced. The temperature is lowered at a temperature drop rate of 0.1 to 20° C./minute, preferably 2 to 10° C./minute.

In the oxygen-amount adjustment process on the sintered body, the atmosphere control in the furnace is also particularly important. Specifically, it is important that gases introduced into the furnace be controlled in a way that the mixing ratio (volume ratio) of oxygen to argon is within a range of O₂/Ar=40/60 to 90/10, and that the gases be introduced into the furnace at a rate of 5 liters/minute or more per 0.1 m³ of the furnace capacity. Such precise adjustments of temperature, atmosphere and time allows production of a sintered body which has the above-described L* value specified in the present invention, and which is useful as an oxide evaporation material.

The heating temperature in the oxygen-amount adjustment process of lower than 900° C. is not preferable. This is because, at such a temperature, dissociation and absorption reactions of oxygen proceed slowly, and it takes long for the reduction process to proceed uniformly to the inside of the sintered body. The temperature exceeding 1100° C. is not preferable. This is because, at such a temperature, oxygen dissociates too rapidly, making impossible the optimal reduction process by the atmosphere. The heating time in the oxygen-amount adjustment process of 10 hour or shorter is not preferable because the reduction process cannot proceed uniformly to the inside of the sintered body. The mixing ratio of gases (O₂/Ar) introduced to the furnace of lower than 40/60 is not preferable. This is because the reduction due to the dissociation of oxygen excessively predominates in such a condition, resulting in a sintered body having an L* value of smaller than 62. Meanwhile, the mixing ratio of gases (O₂/Ar) introduced to the furnace of exceeding 90/10 is not preferable. This is because the oxidization excessively predominates in such a condition, resulting in a sintered body having an L* value exceeding 95.

To obtain the oxide evaporation material of the present invention, it is important to perform an annealing process in a gas atmosphere where an oxygen gas is precisely diluted with an argon gas as described above, that is, in an atmosphere where the oxygen amount is precisely controlled. The atmosphere gases do not always have to be the gas mixture of oxygen and argon. For example, other inert gases such as helium and nitrogen can be used effectively in place of argon. Even air can also be effectively used in place of argon, if the oxygen content in the entire gas mixture is precisely controlled at a constant level. However, if an oxygen gas is introduced into a furnace performing firing with the air as in a conventional technique, the oxygen content of the atmosphere in the furnace cannot be controlled precisely; thus, such atmosphere gases cannot be used effectively. As proposed in the present invention, when a gas mixture having the content ratio of an oxygen gas precisely controlled with an inert gas is introduced to fill a furnace, an oxide evaporation material in an optimal reduction state is obtained.

After the oxygen-amount adjustment process, the temperature is lowered to room temperature at 10° C./minute, and the sintered body can be taken out from the furnace at room temperature. The sintered body thus obtained is processed by, for example, cutting into predetermined dimensions to thereby form an oxide evaporation material. Alternatively, when used is a green compact designed to have predetermined dimensions after the firing in consideration of shrinkage by the sintering, a resultant sintered body can be utilized as the oxide evaporation material without the cutting process after the sintering.

Incidentally, as one method to obtain a high-density sintered body among methods for producing a sputtering target, hot pressing is known to be effective. However, when hot pressing is employed for the materials of the present invention, only obtained is a sintered body having an L* value of 40 or smaller and in an excessively-reduced state. Such a sintered body cannot achieve the object of the present invention.

Meanwhile, the oxide evaporation material of the present invention can be used in the form of cylindrical tablets or pellets having a diameter of 10 to 50 mm and a height of 10 to 50 mm, for example. Furthermore, the oxide evaporation material can be utilized in the form of granules of approximately 1 to 10 mm obtained by grinding the sintered body as described above.

The oxide evaporation material according to the present invention is further allowed to contain, for example, tin, tungsten, molybdenum, zinc, cadmium, and the like as other elements than indium, cerium and oxygen, with the proviso that the characteristics of the present invention are not impaired. However, if the vapor pressure of an oxide of such a metal is much higher than the vapor pressures of indium oxide and cerium oxide, it is preferable not to contain ions of such a metal. This is because the evaporation by various vacuum deposition methods is difficult. For example, oxides of metals such as aluminum, titanium and silicon have vapor pressures much higher than those of indium oxide and cerium oxide. Accordingly, when contained in the oxide evaporation material, such metals are difficult to evaporate together with indium oxide and cerium oxide. For this reason, these metals remain in the oxide evaporation material and are highly concentrated, causing adverse influences such as hindering of evaporation of indium oxide and cerium oxide in the end. Thus, such metals must not be contained.

Moreover, in the oxide evaporation material according to the present invention, cerium may be formed only of an indium oxide phase with an indium site substituted with cerium, or may in a mixture form of a cerium oxide phase and an indium oxide phase with an indium site substituted with cerium. Furthermore, these forms may be mixed with a cerium oxide compound phase and an indium oxide compound phase.

When a high-refractive-index transparent film is formed by various vacuum deposition methods using the oxide evaporation material of the present invention, the oxygen content in the oxide evaporation material is already optimally adjusted. Accordingly, even when a small amount of oxygen is introduced into a film-formation vacuum chamber, a transparent film having optimal oxygen deficiency is obtained. This brings about advantages that the difference in composition is made small between the transparent film and the oxide evaporation material, reducing the influence on the characteristic variation caused by the change in the amounts of oxygen introduced.

(2) High-Refractive-Index Transparent Film

A high-refractive-index transparent film of indium oxide containing cerium can be formed by various vacuum deposition methods such as electron beam deposition, ion plating, and high-density plasma-assist evaporation, using the oxide evaporation material according to the present invention, the oxide evaporation material being formed of the sintered body containing indium oxide as the main component and cerium, having a cerium content of more than 0.110 and equal to or less than 0.538 in the Ce/In atomic ratio, and having an L* value in the CIE 1976 color space of 62 to 95.

The high-refractive-index transparent film may be an amorphous film, a crystalline film, or a film made of a mixture of these. However, being a crystalline film, a high-refractive-index transparent film having a high strength and excellent adhesiveness is obtained. The crystalline film can be obtained by heating a substrate at 200° C. or higher during the film formation. Alternatively, the crystalline film can be obtained by a method in which a film obtained without heating the substrate is annealed at 200° C. or higher.

The high-refractive-index transparent film according to the present invention can be formed from the oxide evaporation material that has a composition slightly different from that of the film. Accordingly, the high-refractive-index transparent film is an indium oxide film containing cerium with the Ce/In atomic ratio of more than 0.110 and equal to less than 0.538. If the cerium content (Ce/In atomic ratio) is lower than 0.110, a high-refractive-index film having a refractive index of 2.15 or higher at a wavelength of 550 nm cannot be obtained as described above. Since the upper limit of the cerium content in the oxide evaporation material according to the present invention is 0.538 as described above, the upper limit of the cerium content in the high-refractive-index transparent film is also 0.538. The high-refractive-index transparent film containing a larger amount of cerium has a higher refractive index, and thus is useful as an optical film.

The high-refractive-index transparent film according to the present invention may contain, for example, tin, tungsten, molybdenum, zinc, cadmium, and the like as other elements than indium, cerium and oxygen, with proviso that the characteristics of the present invention are not impaired, as in the above-described oxide evaporation material. The high-refractive-index transparent film may have a conductivity.

Examples of the present invention are described below in detail.

Examples 1 to 4 Manufacturing of Oxide Evaporation Material

An I₂O₃ powder having an average particle diameter of 0.8 μm and a CeO₂ powder having an average particle diameter of 1 μm were used as raw-material powders. These In₂O₃ and CeO₂ powders were blended in such a manner that the Ce/In atomic ratio was to be 0.142. The powders were put in a resin pot, and mixed with a wet ball mill. At this time, hard ZrO₂ balls were used, and the mixing time was 20 hours.

After the mixing, the slurry was taken out, and a polyvinyl alcohol binder was added to the obtained slurry. The mixture was dried with a spray dryer or the like, and granulated.

The granulated product was subjected to uniaxial press molding at a pressure of 1 ton/cm². Thus, a cylindrical green compact having a diameter of 30 mm and a thickness of 40 mm was obtained.

Next, the obtained green compact was sintered as follows.

Specifically, the green compact was heated at 300° C. in the air in a sintering furnace for approximately 10 hours to remove the binder from the green compact. Then, the temperature was increased at a rate of 1° C./minute in an atmosphere into which oxygen was introduced at a rate of 5 liters/minute per 0.1 m³ of the furnace capacity. At 1250° C., the resulting green compact was sintered for 2 hours (pressureless sintering). Oxygen was continuously introduced, also during the cooling after the sintering where the temperature was lowered to 1000° C. at 10° C./minute.

Next, the introduced gas was switched to a gas mixture of oxygen and argon, and heating was performed at 1000° C. and kept for 15 hours (hereinafter, this process is referred to as an oxygen-amount adjustment process on the sintered body). Thereafter, the temperature was lowered at 10° C./minute to room temperature.

By changing the mixing ratio of oxygen to argon in the gas mixture, oxide sintered bodies (oxide evaporation materials) of various L* values were obtained.

Specifically, the oxide evaporation material according to Example 1 was manufactured under the condition where the flow ratio (i.e., volume ratio) of oxygen gas/argon gas was “40/60.” The oxide evaporation material according to Example 2 was manufactured under the condition where the volume ratio was “60/40.” The oxide evaporation material according to Example 3 was manufactured under the condition that the volume ratio was “80/20.” The oxide evaporation material according to Example 4 was manufactured under the condition that the volume ratio was “90/10.”

Note that the volumes and the weights of the obtained oxide sintered bodies (oxide evaporation materials) were measured to calculate the densities which were 5.4 to 5.8 g/cm³. Additionally, the fractured surfaces of the oxide sintered bodies were observed with a scanning electron microscope to obtain the average diameters of 100 crystal particles in the oxide sintered bodies. It was found out that any of the oxide sintered bodies had an average diameter of 2 to 7 μm. Moreover, by the four-point probe method with a resistivity meter, the sheet resistances were measured on surfaces of the oxide sintered bodies to be radiated with electron beam to calculate the specific resistances. It was found out that the sheet resistances were 1 kΩcm or lower. Furthermore, composition analysis was performed on all of the oxide sintered bodies by ICP optical emission spectrometry. It was found out that the oxide sintered bodies had the composition thus fed. In addition, the L* values in the CIE 1976 color space of the surfaces and the insides of the sintered bodies were measured with a color difference meter (spectro guide, E-6834 manufactured by BYK-Gardner GmbH). The values were shown to be almost the same.

Table 1a shows the flow ratios (i.e., volume ratios) of oxygen gas/argon gas in the gas mixtures introduced in the oxygen-amount adjustment process on the sintered bodies, and the L* values of the obtained oxide sintered bodies (oxide evaporation materials).

TABLE 1a L* value Optimal of oxide oxygen evaporation mixed A B material amount Example:  1 0.142 40/60 62  9  2 0.142 60/40 86  8  3 0.142 80/20 91  5  4 0.142 90/10 95  3 Comparative Example:  1 0.142 30/70 58 20  2 0.142 100/0   98  0  3 0.142 — 50 36 Example:  5 0.346 40/60 65  8  6 0.346 60/40 87  7  7 0.346 80/20 90  4  8 0.346 90/10 94  3 Comparative Example:  4 0.346 30/70 57 18  5 0.346 100/0   99  0  6 0.346 — 48 45 Example:  9 0.110 60/40 84  7 10 0.202 60/40 85  7 11 0.538 60/40 84  6 Comparative Example:  7 0.080 60/40 84  6 *(Remarks) A: Ce/In atomic ratio of oxide evaporation material, B: O₂/Ar flow ratio in oxygen-amount adjustment for sintered body

TABLE 1b Film characteristics with optimal oxygen mixed amount Transmittance of film itself Refractive Ce/In Film Film Fracture in index at a atomic adhesive formation on oxide visible wavelength Film ratio force to speed evaporation region % of 550 nm crystalline of film substrate (nm/min) material Example: 1 91 2.20 crystalline 0.142 strong 155 no film fracture 2 91 2.20 crystalline 0.142 strong 154 no film fracture 3 91 2.20 crystalline 0.142 strong 153 no film fracture 4 91 2.20 crystalline 0.142 strong 152 no film fracture Comparative Example: 1 91 2.28 crystalline 0.185 weak 155 no film fracture 2 91 2.20 crystalline 0.142 strong 85 no film fracture 3 91 2.28 crystalline 0.230 weak 156 fracture film Example: 5 91 2.42 crystalline 0.346 strong 156 no film fracture 6 91 2.42 crystalline 0.346 strong 155 no film fracture 7 91 2.42 crystalline 0.346 strong 154 no film fracture 8 91 2.42 crystalline 0.346 strong 153 no film fracture Comparative Example: 4 91 2.42 crystalline 0.398 weak 156 no film fracture 5 91 2.42 crystalline 0.346 strong 95 no film fracture 6 91 2.42 crystalline 0.456 weak 157 fracture film Example: 9 91 2.15 amorphous 0.108 strong 160 no fracture 10  91 2.30 amorphous 0.204 strong 158 no fracture 11  90 2.51 amorphous 0.539 strong 138 no fracture Comparative Example: 7 91 2.10 amorphous 0.080 strong — fracture

Forming of High-Refractive-Index Transparent Film, Film Characteristics Evaluation, and Film Formation Evaluation

(1) For forming of a high-refractive-index transparent film, a magnetic field deflection-type electron beam evaporator was used.

The evacuation system includes a low evacuation system with a rotary pump and a high evacuation system with a cryopump. The evacuation is possible to 5×10⁻⁵ Pa. Electron beams are generated by heating a filament, and accelerated by the electric field applied between the cathode and the anode. The electron beams are curved in the magnetic field of the permanent magnet, and then radiated to an oxide evaporation material placed in a tungsten crucible. The intensity of the electron beams can be adjusted by changing the voltage applied to the filament. Moreover, the radiation position of the beams can be changed by changing the acceleration voltage applied between the cathode and the anode.

Films were formed under the following conditions.

An Ar gas and an O₂ gas were introduced into the vacuum chamber and the pressure was kept at 1.5×10⁻² Pa. In this event, a high-refractive-index transparent film is obtained by changing the mixing ratio of the Ar gas to the O₂ gas introduced into the vacuum chamber. Then, the characteristics of the high-refractive-index transparent film were evaluated. Each of the cylindrical oxide evaporation materials of Examples 1 to 4 was disposed standing in the tungsten crucible. Electron beams were radiated to a central portion of the circular surface of the oxide evaporation material, so that a high-refractive-index transparent film having a thickness of 200 nm was formed on a Corning 7059 glass substrate having a thickness of 1.1 mm. The electron gun was set at a voltage of 9 kV and a current value of 150 mA. The substrate was heated at 350° C.

(2) The characteristics of the thin films (high-refractive-index transparent films) thus obtained were evaluated in the following procedure.

First, the thickness each of thin films (high-refractive-index transparent films) was evaluated based on the measurement, with a contact profilometer (manufactured by KLA-Tencor Corporation), based on the difference in step height between portions where the film was formed and not formed to calculate the film formation speed.

Next, the transmittance of the film including the glass substrate (the glass substrate B with the film L) [T_(L+B) (%)] was measured with a spectrophotometer (U-4000 manufactured by Hitachi, Ltd.). The transmittance of only the glass substrate (the glass substrate B) [T_(B) (%)] was also measured by the same method to calculate the transmittance of the film itself by [T_(L+B)=T_(B)]×100(%).

The crystalline of the film was evaluated by X-ray diffraction measurement. As the X-ray diffractometer, X 'Pert PRO MPD (manufactured by PANalytical B.V.) was used. The measurement conditions were set in a wide range, and CuKα ray was used. The measurement was performed with a voltage of 45 kV and a current of 40 mA. The film crystalline was evaluated according to the presence or absence of an X-ray diffraction peak. This result is also shown in the column of “film crystalline” in Table 1b.

Next, the composition (Ce/In atomic ratio) of the film was measured by ICP optical emission spectrometry. The adhesive force of the film to the substrate was evaluated based on JIS C0021. The evaluation was made: favorable (strong) when the film was not peeled off; and not sufficient (weak) when the film was peeled off. These results are also shown in the columns of “Ce/In atomic ratio of film” and “film adhesive force to substrate” in Table 1b.

The transmittance and the film formation speed of each thin film (high-refractive-index transparent film) were dependent on the mixing ratio of the O₂ gas to the Ar gas introduced into the film-formation vacuum chamber during the film formation. The mixing ratio of the O₂ gas [O₂/(Ar+O₂)(%)] was changed from 0 to 50% in units of 1%. As the amount of oxygen introduced was increased, the transmittance was increased; however, when the amount of oxygen introduced reached a certain level or higher, the transmittance no longer changed. Meanwhile, when oxygen was introduced excessively, the film formation speed was decreased. The mixing ratio of the O₂ gas at which the highest transmittance characteristics and the maximum film formation speed were obtained was determined as the optimal oxygen mixed amount. This result is shown in the column of “optimal oxygen mixed amount” in Table 1a.

A thin film (high-refractive-index transparent film) formed with an oxygen amount smaller than the optimal oxygen mixed amount was not only poor in conductivity but also low in transmittance in the visible region. Meanwhile, a thin film (high-refractive-index transparent film) formed with the optimal oxygen mixed amount was not only excellent in conductivity but also high in transmittance in the visible region.

(3) Using each oxide evaporation material of Examples 1 to 4, the film formation evaluation was performed. In this evaluation, obtained were: the average transmittance of the film itself in the visible region (wavelength of 400 to 800 nm) and the refractive index at a wavelength of 550 nm, in the case where the film has the optimal oxygen mixed amount.

These evaluation results are shown in the columns of “transmittance of film itself in visible region %” and “refractive index at a wavelength of 550 nm” in Table 1b.

In the film formation using the oxide evaporation materials of Examples 1 to 4, the optimal oxygen mixed amount to be introduced into the film-formation vacuum chamber so as to obtain a high-refractive-index transparent film having a high transmittance was considerably small. This is because each oxide evaporation material contained the optimal oxygen amount. Additionally, the film formed with the optimal oxygen mixed amount had the same composition as that of the oxide evaporation material, and exhibited a high transmittance in the visible region. The refractive index at a wavelength of 550 nm was 2.20 and was higher than those of the conventional materials such as ITO. Moreover, the film was observed to be a crystalline film of indium oxide with the bixbyite structure, and also had such a strong adhesive force to the substrate that the film can be used practically.

Furthermore, an electron gun was set at a voltage of 9 kV and a current value of 150 mA. After the radiation of electron beams for 60 minutes, the oxide evaporation material was visually observed to check whether the oxide evaporation material had a fracture or a crack (durability test on oxide evaporation material). The oxide evaporation materials of Examples 1 to 4 had no crack generated even after being continuously used (evaluated as “no fracture”).

Such high-refractive-index transparent film can be said to be useful for forming an anti-reflection film or the like in combination with a low-refractive-index film.

Comparative Examples 1 and 2

Oxide sintered bodies were produced as in the Examples 1 to 4 except that the mixing ratio of the introduced gases in the oxygen-amount adjustment process on the sintered bodies was changed. Specifically, the O₂/Ar flow ratio was 30/70 in Comparative Example 1, and 100/0 in Comparative Example 2. The density, specific resistance, crystal particle diameter, and composition of the obtained sintered bodies were evaluated in the same way as above. Any of these parameters were equivalent to those of Examples 1 to 4. The colors of the surface and the inside each of the obtained oxide sintered bodies were the same. The L* value was measured, and shown in Table 1a.

Next, the film formation evaluation was performed as in Examples 1 to 4.

The result is also shown in Tables 1a and 1b above.

The oxide evaporation material of Comparative Example 1 had an L* value (58) smaller than the specified range of the present invention (62 to 95), and is characterized in that the optimal oxygen mixed amount (20) during the film formation was larger than those of the oxide evaporation materials of Examples 1 to 4. As to the film characteristics with the optimal oxygen mixed amount, the transmittance was equivalent to those of Examples 1 to 4. However, the adhesive force to the substrate was weaker than those of Examples 1 to 4. This seems to be caused by a larger amount of oxygen introduced during the film formation. Moreover, the film obtained from the oxide evaporation material greatly differed in composition from the evaporation material (the Ce/In atomic ratio of the oxide evaporation material was 0.142, whereas the value of the high-refractive-index transparent film obtained therefrom was 0.185). Accordingly, the refractive index also slightly differed from those of Examples 1 to 4. This indicates that it is difficult to design the film composition. Furthermore, since a larger amount of oxygen has to be introduced into the film-formation vacuum chamber, when the oxide evaporation material is used for the mass production of the films, the compositions and the characteristics of the films greatly vary from one another due to the influence of the change in the oxygen concentration in the vacuum chamber. Thus, it can be concluded that the oxide evaporation material of Comparative Example 1 is inappropriate for forming films in large quantities.

Meanwhile, Comparative Example 2 is an example of the oxide evaporation material having an L* value (98) larger than the specified range of the present invention. The optimal oxygen mixed amount during the film formation was 0%, and the film had a refractive index equivalent those of Examples 1 to 4. However, the film formation speed (85 nm/min) was lower than those of Examples 1 to 4. This is because the oxide evaporation material contained a large amount of oxygen, making the sublimation difficult.

Thus, it can be concluded that such an oxide evaporation material is inappropriate for forming films in large quantities.

Comparative Example 3

Next, a sintered body of indium oxide containing cerium was produced according to the production technique for a sintered body of a sputtering target proposed in Patent Document 3.

First, an In₂O₃ powder having an average particle diameter of 1 μm or smaller and a CeO₂ powder having an average particle diameter of 1 μm or smaller were used as raw-material powders. These In₂O₃ and CeO₂ powders were blended in such a manner that the Ce/In atomic ratio was to be 0.008. The powders were put in a resin pot, and mixed with a wet ball mill. At this time, hard ZrO₂ balls were used, and the mixing time was 20 hours. After the mixing, the slurry was taken out, filtrated, dried, and then granulated. The granulated powder thus obtained was subjected to cold isostatic pressing at 3 t/cm² for molding.

The obtained green compact was put in a sintering furnace into which oxygen was introduced at a rate of 5 liters/minute per 0.1 m³ of the furnace capacity to create an atmosphere. The green compact was sintered at 1450° C. for 8 hours. In this event, the temperature was increased to 1000° C. at 1° C./minute, and from 1000 to 1450° C. at 2° C./minute. Then, the introduction of oxygen was terminated, and the temperature was lowered from 1450 to 1300° C. at 5° C./minute. The temperature was kept at 1300° C. for 3 hours in an atmosphere into which an argon gas was introduced at a rate of 10 liters/minute per 0.1 m³ of the furnace capacity. Thereafter, a sintered body was left to cool.

The obtained sintered body was processed into a cylindrical shape having a diameter of 30 mm and a thickness of 40 mm. The sintered body had a density of 6.5 g/cm³ and a specific resistance of 1.5 mQcm. The crystal particle diameter was 9 to 12 μm. The composition was substantially the same as the composition thus fed. The colors of the surface and the inside of the obtained sintered body were the same. The L* value was measured, and it was a considerably small value (50) as shown in Table 1a. This indicates that the amount of oxygen in the oxide evaporation material was considerably small.

Next, the film formation evaluation was performed as in Examples 1 to 4.

The result is also shown in Tables 1a and 1b above.

The oxide evaporation material of Comparative Example 3 had an L* value (50) substantially smaller than the specified range of the present invention (62 to 95), and is characterized in that the optimal oxygen mixed amount (36) during the film formation was substantially larger than those of the oxide evaporation materials of Examples 1 to 4. As to the film characteristics with the optimal oxygen mixed amount, the transmittance was equivalent to those of the oxide evaporation materials of Examples 1 to 4 having the same composition. However, the refractive index was slightly different from those of Examples 1 to 4. This seems to be caused by the large difference in composition between the film and the oxide evaporation material. Furthermore, the film of Comparative Example 3 had an adhesive force to the substrate weaker than those of Examples 1 to 4. This seems to be caused by a larger amount of oxygen introduced during the film formation. Such an oxide evaporation material greatly differs in composition from the film to be obtained, accordingly making it difficult to design the film composition. Moreover, since a larger amount of oxygen has to be introduced into the film-formation vacuum chamber, when the oxide evaporation material is used for the mass production of the films, the compositions and the characteristics of the films greatly vary from one another due to the influence of the change in the oxygen concentration in the vacuum chamber. In addition, the durability test was performed on the oxide evaporation material under the same conditions as those in Examples 1 to 4. It was found out that the oxide evaporation material had a crack generated after films are formed continuously (evaluated as “fracture”). If films were continuously formed using such an oxide evaporation material having a crack, problems occur such as a large variation in the film formation speed, hindering stable film formation.

Thus, it can be concluded that the oxide evaporation material of Comparative Example 3 is inappropriate for forming films in large quantities.

Examples 5 to 8

Oxide sintered bodies (oxide evaporation materials) of Examples 5 to 8 were produced under exactly the same conditions as those of Examples 1 to 4 including the conditions for the oxygen-amount adjustment for the sintered bodies, except that the In₂O₃ and CeO₂ powders were blended in such a manner that the Ce/In atomic ratio was to be 0.346.

Specifically, the oxide evaporation material according to Example 5 was manufactured under the condition where the flow ratio (i.e., volume ratio) of oxygen gas/argon gas was “40/60.” The oxide evaporation material according to Example 6 was manufactured under the condition where the volume ratio was “60/40.” The oxide evaporation material according to Example 7 was manufactured under the condition that the volume ratio was “80/20.” The oxide evaporation material according to Example 8 was manufactured under the condition that the volume ratio was “90/10.”

Then, the density, specific resistance, crystal particle diameter, and composition of the obtained oxide sintered bodies (oxide evaporation materials) of Examples 5 to 8 were evaluated in the same way as above. Any of these parameters were equivalent to those of Examples 1 to 4. Moreover, the colors of the surface and the inside each of the obtained oxide sintered bodies were the same. Table 1a shows the measurement results of the L* value.

Next, the film formation evaluation was performed as in Examples 1 to 4.

The result is also shown in Tables 1a and 1b above.

In the film formation using the oxide evaporation materials of Examples 5 to 8 the optimal oxygen mixed amount to be introduced into the film-formation vacuum chamber so as to obtain a high-refractive index transparent film having the lowest resistance and the highest transmittance was, as in Examples 1 to 4, considerably small. This is because each oxide evaporation material contained the optimal oxygen amount. Additionally, the film formed with the optimal oxygen mixed amount had the same composition as that of the oxide evaporation material, had a high refractive index of 2.42 at a wavelength of 550 nm and exhibited a high transmittance also in the visible region. Moreover, all the films are crystalline films of indium oxide with the bixbyite structure, and also had such a strong adhesive force to the substrate that the films can be used practically. Furthermore, the oxide evaporation materials of Examples 5 to 8 had no crack generated even when continuously used.

Such high-refractive-index transparent films can be said to be useful for forming an anti-reflection film and the like in combination with a low-refractive-index film.

Comparative examples 4 and 5

Oxide evaporation materials of Comparative Examples 4 and 5 were manufactured under the same conditions as those of Comparative Examples 1 and 2, except that the In₂O₃ and CeO₂ powders were blended in such a manner that the Ce/In atomic ratio was to be 0.346. Specifically, as to the condition for the oxygen-amount adjustment for the sintered bodies, the O₂/Ar flow ratio was 30/70 in Comparative Example 4, and 100/0 in Comparative Example 5. The density, specific resistance, crystal particle diameter, and composition of the obtained sintered bodies were evaluated in the same way as above. Any of these parameters were equivalent to those of Examples 5 to 8. The colors of the surface and the inside each of the obtained oxide sintered bodies were the same. The L* value was measured, and shown in Table 1a.

Next, the film formation evaluation was performed as in Examples 1 to 4.

The result is also shown in Tables 1a and 1b.

The oxide evaporation material of Comparative Example 4 had an L* value (57) smaller than the specified range of the present invention (62 to 95). The optimal oxygen mixed amount during the film formation was large when compared with cases where the oxide evaporation materials of Examples 5 to 8 were used. As to the film characteristics with the optimal oxygen mixed amount, the transmittance was substantially equivalent to those of Examples 5 to 8. However, the adhesive force to the substrate was weaker than those of Examples 5 to 8. This is because a larger amount of oxygen was introduced during the film formation. Additionally, the film obtained from this oxide evaporation material greatly differed in composition from the evaporation material. Accordingly, the refractive index was also slightly different from those of Examples 5 to 8. This indicates that it is difficult to design the film composition. Moreover, since a larger amount of oxygen has to be introduced into the film-formation vacuum chamber, when the oxide evaporation material is used for the mass production of the films, the compositions and the characteristics of the films greatly vary from one another due to the influence of the change in the oxygen concentration in the vacuum chamber. Thus, it can be concluded that the oxide evaporation material of Comparative Example 4 is also inappropriate for forming films in large quantities.

Meanwhile, Comparative Example 5 is an example of the oxide evaporation material having an L* value (99) larger than the specified range of the present invention. The optimal oxygen mixed amount during the film formation was 0%, and the film had a refractive index equivalent those of Examples 5 to 8. However, the film formation speed was lower than those of Examples 5 to 8. This is because the oxide evaporation material contained a large amount of oxygen, making the sublimation difficult. Thus, such an oxide evaporation material is inappropriate for forming films in large quantities.

Comparative Example 6

An oxide evaporation material of Comparative Example 6 was manufactured under the same conditions as those of Comparative Example 3, except that the In₂O₃ and CeO₂ powders were blended in such a manner that the Ce/In atomic ratio was to be 0.346. The density, specific resistance, crystal particle diameter, and composition of the obtained sintered body were evaluated in the same way as above. Any of these parameters were equivalent to those of Comparative Example 3. The colors of the surface and the inside of the obtained oxide sintered body were the same. The L* value was measured, and shown in Table 1a.

Next, the film formation evaluation was performed as in Examples 1 to 4.

The result is also shown in Tables 1a and 1b above.

The oxide evaporation material of Comparative Example 6 also had an L* value (48) substantially smaller than the specified range of the present invention, and is characterized in that the optimal oxygen mixed amount (45) during the film formation was substantially larger than those of the oxide evaporation materials of Examples 5 to 8. As to the film characteristics with the optimal oxygen mixed amount, the transmittance was equivalent to those of the oxide evaporation materials of Examples 5 to 8 having the same composition. However, the refractive index was slightly different from those of Examples 5 to 8. This seems to be caused by the large difference in composition between the film and the oxide evaporation material. Furthermore, the film of Comparative Example 6 had an adhesive force to the substrate weaker than those of Examples 5 to 8. This seems to be caused by a larger amount of oxygen introduced during the film formation. Such an oxide evaporation material greatly differs in composition from the film to be obtained, accordingly making it difficult to design the film composition. Moreover, since a larger amount of oxygen has to be introduced into the film-formation vacuum chamber, when the oxide evaporation material is used for the mass production of the films, the compositions and the characteristics of the films greatly vary from one another due to the influence of the change in the oxygen concentration in the vacuum chamber. In addition, the durability test was performed on the oxide evaporation material under the same conditions as those in Examples 1 to 4. It was found out that the oxide evaporation material had a crack generated after films are formed continuously (evaluated as “fracture”). If films were continuously formed using such an oxide evaporation material having a crack, problems occur such as a large variation in the film formation speed, hindering stable film formation.

Thus, it can be concluded that the oxide evaporation material of Comparative Example 6 is also inappropriate for forming films in large quantities.

Examples 9 to 11 and Comparative Example 7

Oxide sintered bodies (oxide evaporation materials) of Examples 9 to 11 and Comparative Example 7 were produced under the same conditions as those of Example 2 (that include the condition for the flow ratio of oxygen gas/argon gas of “60/40”), except that the I₂O₃ and CeO₂ powders were blended in such a manner that the Ce/In atomic ratio was: 0.110 (Example 9), 0.202 (Example 10), 0.538 (Example 11), and 0.080 (Comparative Example 7).

Then, the density, specific resistance, crystal particle diameter, and composition of the obtained oxide sintered bodies (oxide evaporation materials) of Examples 9 to 11 and Comparative Example 7 were evaluated in the same way as above. Any of these parameters were equivalent to those of Example 2. Moreover, the colors of the surface and the inside each of the obtained oxide sintered bodies were substantially equivalent. Table 1a shows the measurement results of the L* value.

The film formation evaluation was performed as in Examples 1 to 4.

The result is also shown in Tables 1a and 1b.

In the film formation using the oxide evaporation materials of Examples 9 to 11 and Comparative Example 7, the optimal oxygen mixed amount to be introduced into the film-formation vacuum chamber so as to obtain a high-refractive-index transparent film having the lowest resistance and the highest transmittance was considerably small as in Examples 1 to 4. This is because each oxide evaporation material contained the optimal oxygen amount. Additionally, the film formed with the optimal oxygen mixed amount had the same composition as that of the oxide evaporation material, and exhibited a high refractive index (except for Comparative Example 7) and a high transmittance also in the visible region. Moreover, all the films were observed to be amorphous films, and also had such a strong adhesive force to the substrate that the films can be used practically. Furthermore, the durability test was performed on the oxide evaporation materials under the same conditions as those in Examples 1 to 4. It was found out that the oxide evaporation materials had no crack generated even when continuously used, except for Comparative Example 7.

The refractive indexes of the films formed from the oxide evaporation materials of Examples 9 to 11 were 2.15 to 2.51 at a wavelength of 550 nm, and higher than the refractive index (1.9 to 2.1) of the conventional ITO film as described above. This brings about advantages that optical design is easy to produce an optical effect such as optical interference, and the like when the film is stacked with a low-refractive-index film. Furthermore, such oxide evaporation materials are suitable for the mass production of films, and can be said to be superior in mass production of high-quality high-refractive-index transparent films by vacuum deposition methods.

However, the refractive index of the film formed from the oxide evaporation material of Comparative Example 7 was 2.10 at a wavelength of 550 nm and substantially equivalent to that of the conventional ITO film. Accordingly, the oxide evaporation material was not as superior as the oxide evaporation materials of Examples 9 to 11.

Comparative Example 8

Next, a sintered body of indium oxide containing cerium was produced according to the production technique for a sintered body of a sputtering target proposed in Patent Document 2.

An In₂O₃ powder having an average particle diameter of approximately 2 μm and the CeO₂ powder having an average particle diameter of approximately 2 μm were used as raw-material powders. In addition, an SnO₂ powder having an average particle diameter of approximately 2 μm and a TiO₂ powder having an average particle diameter of approximately 2 μm were used as sintering aids. The In₂O₃ and CeO₂ powders were blended in such a manner that the Ce/In atomic ratio was to be 0.142. Furthermore, the SnO₂ and Tio₂ powders were blended in such a manner that a Sn/In atomic ratio was to be 0.01, and that a Ti/In atomic ratio was to be 0.005. These mixtures were put in a resin pot, and mixed with a wet ball mill. At this time, hard ZrO₂ balls were used, and the mixing time was 24 hours. After the mixing, the slurry was taken out, filtrated, dried, and then granulated. The granulated powder thus obtained was subjected to cold isostatic pressing at 500 Kgf/cm². Thus, a green compact having a density of 3.5 g/cm³ was obtained.

The obtained green compact was put in a sintering furnace into which oxygen was introduced at a rate of 5 liters/minute per 0.1 m³ of the furnace capacity to create an atmosphere. The green compact was sintered at 1450° C. for 40 hours. In this event, the temperature was increased to 1000° C. at 1° C./minute, and from 1000 to 1450° C. at 2° C./minute. Then, the introduction of oxygen was terminated, and the temperature was lowered from 1450 to 1300° C. at 5° C./minute. The temperature of 1300° C. was kept for 3 hours in an atmosphere into which an argon gas was introduced at a rate of 10 liters/minute per 0.1 m³ of the furnace capacity. Thereafter, a sintered body was left to cool.

The obtained sintered body was processed into a cylindrical shape having a diameter of 30 mm and a thickness of 40 mm. The sintered body had a density of 6.5 g/cm³ and a specific resistance of 0.9 mΩcm. The crystal particle diameter was 11 to 13 μm. The composition was substantially the same as the composition thus fed. The colors of the surface and the inside of the obtained sintered body were substantially the same. The L* value was measured, and it was a considerably small value of “44.” This indicates that the amount of oxygen in the oxide evaporation material was considerably small.

The same film formation evaluation as in Examples 1 to 4 was performed on the oxide evaporation material (oxide sintered body) of Comparative Example 8 thus manufactured.

As a result, the film formation speed changed rapidly, and the film was not formed stably. From the observation of the oxide evaporation material after the film formation test, it was found out that a solid material was attached to the surface of the oxide evaporation material. The analysis on this solid material revealed that the solid material was concentrated mass of titanium oxide. Titanium oxide was increased in amount and remained on the surface when the film formation was continued presumably because the high vapor pressure of titanium oxide makes it difficult to evaporate. Such a titanium oxide mass covering the surface of the oxide evaporation material hinders stable film formation by vacuum deposition.

From the above, even if a sintered body of indium oxide containing cerium is produced by adopting the production technique for a sintered body of a sputtering target proposed in Patent Document 2, the sintered body cannot be utilized as an oxide evaporation material that can be stably employed in vacuum deposition methods.

POSSIBILITY OF INDUSTRIAL APPLICATION

The use of the oxide evaporation material according to the present invention enables formation by vacuum deposition methods of a high-refractive-index transparent film having a low resistance and a high transmittance in the visible to near-infrared regions. Accordingly, it has a possibility of industrial applicability that it is utilized as an oxide evaporation material for forming an anti-reflection film and the like in combination with a low-refractive-index film. 

1. An oxide evaporation material comprising a sintered body containing: indium oxide as a main component thereof; and cerium, wherein a cerium content is more than 0.110 and equal to or less than 0.538 in a Ce/In atomic ratio, and an L* value in a CIE 1976 color space is 62 to
 95. 2. A high-refractive-index transparent film formed by electron beam deposition, ion plating, or high-density plasma-assist evaporation, using an oxide evaporation material comprising a sintered body containing: indium oxide as a main component thereof; and cerium, the oxide evaporation material having a cerium content of more than 0.110 and equal to or less than 0.538 in a Ce/In atomic ratio and an L* value of 62 to 95 in a CIE 1976 color space, the high-refractive-index transparent film having a cerium content of more than 0.110 and equal to or less than 0.538 in the Ce/In atomic ratio, and having a refractive index of 2.15 to 2.51 at a wavelength of 550 nm. 